1. Field of the Invention
The invention relates to the field of structures and method for supporting photoresist covered transparent substrates during exposure of the photoresist to photoresist-activating radiation.
2. Prior Art
In order to obtain high yields of high quality operative CMOS/SOS devices, it has been found necessary to protect the edges of silicon islands on a sapphire substrate and the portions of the substrate which are not covered by the silicon from interacting with chemicals and ambients utilized during the fabrication of active devices in the silicon. Such protection can be provided by a layer of silicon oxide overlying the edges of the silicon islands and the sapphire between the islands.
One prior art technique for coating a silicon island with silicon oxide is to thermally grow oxide in an oxidizing ambient atmosphere, thereby consuming silicon of the island to form the silicon oxide. Unfortunately, oxide grown by this technique is undercut at the interface between the sapphire substrate and the silicon oxide. This results in overcoatings of other materials being discontinuous and/or having voids within the undercuts. Where the overcoating material is intended to form circuits by electrically interconnecting spaced-apart portions of the structure, electrical opens which result from the discontinuous character of the overcoating render the circuits inoperative. Where voids occur within the undercuts, subsequent processing solutions and atmospheres can enter the voids. Trapping of processing solutions in the voids adversely effects the useful life of the circuits.
Another prior art technique for coating the edges of the silicon islands with protective silicon oxide is to deposit a layer of silox over the tops and edges of the islands and on the portion of the sapphire substrate which is exposed between the islands. Silox is formed by the interaction of silane (SiH.sub.4) and oxygen (O.sub.2) in the vicinity of the silicon-on-sapphire wafer. The silicon of the islands is not intentionally consumed to produce the deposited silox. Subsequently, the silox is coated with a layer of negative photoresist. After the photoresist is prebaked the wafer is placed photoresist coating down on top of a wafer chuck in preparation for activation of the photoresist. The photoresist is activated by exposure to photoresist-activating radiation such as ultra-violet light which is projected through the transparent substrate. The activating radiation is chosen to have a wavelength which is absorbed by the silicon islands but not by the substrate or the silox. Thus, the silicon islands are self masking i.e. they block the radiation thereby preventing exposure of the photoresist thereover. Thus, the photoresist should be exposed only in the areas between islands. Subsequently, the photoresist is developed and is utilized as a mask for the etching of the silox layer to expose the top (or upper) surface of the islands without exposing the edges of the islands or the substrate. This prior art technique is subject to sporatic and spotty results as a result of wafer to wafer variations in the quality of the exposure. Underexposure of the photoresist results in the silox being removed from the edges of the islands, thus defeating the purpose of the protective oxide back fill. Overexposure of the photoresist usually results in silox remaining on at least a portion of the upper surface of the islands. Silox extending over the upper surface of the island can adversely effect the quality of devices formed in the island and can prevent the formation of operative devices. Utilizing this technique, the latitude or degree of variation in the itensity and duration of exposure of the wafer to photoresist-activating radiation which results in proper exposure of the photoresist is small. As a result, the quality of the exposure varies from wafer to wafer. In some instances, some portions of a wafer may be underexposed while other portions are overexposed. A further problem with this technique is the fact that the photoresist coating is sometimes scratched by the wafer chuck, with the result that the photoresist does not protect the silox everywhere it should.
As a result of the prior art difficulties with achieving proper exposure of the photoresist, the silox backfill layers are protected by photoresist during etching of silicon oxide only while the silox back-fill is initially being defined. As a consequence, each time silicon oxide over the islands is etched, part of the backfill is etched away. If a sufficient quantity of the backfill is etched away in this manner, some of the problems which the backfill is intended to prevent can occur.
Thus, a technique is needed which will assure proper exposure of the photoresist thereby assuring that the edges of the islands will be protected by the back-fill silox and that the upper surface of the islands will be substantially uniformly and completely free of silox and thus exposed to subsequent processing environments. Further, a technique is needed which will allow the back-fill silox to be protected during each etching of silicon oxide, not just during the etch which defines the back-fill.